A system and method for determining the pH value of a liquid. The system and method excite the liquid molecules in the liquid by light with a predetermined wavelength and an intensity component, transmitted through the liquid in a predetermined wavelength range, of the light used for excitation is captured. This intensity component is used to produce a wavelength/intensity absorption characteristic and this absorption characteristic is related to a reference absorption characteristic associated with a specific pH value of the liquid. The system and method thus can be used to obtain drinking water from a fuel for aircraft.